1. Field of the Invention
The present invention relates to an optical element for constituting an optical system using ultraviolet light, particularly KrF excimer laser light, ArF excimer laser light or the like, and also to a method and apparatus for producing such an optical element.
2. Related Background Art
Light in an ultraviolet region is conventionally used in optical apparatus such as a projection light-exposure apparatus for use in semiconductor device production. As a laser for generating light with a wavelength of 250 nm or less, lasers such as KrF, KrCl, ArF, ArCl, Xe.sub.2, Kr.sub.2, Ar.sub.2, and F.sub.2 lasers are available. Fluorite exhibits a high transmission in the ultraviolet region, and is widely used to constitute optical elements such as a mirror, lens, beam splitter, etc., which constitute an optical system used in a projection light-exposure apparatus.
For example, Japanese Patent Application Laid-Open Nos. 7-261002, 7-218701, and 8-220304, disclose a structure of antireflection film suitable for an optical element. In the optical element for use in a KrF excimer laser, a multilayer is used as an antireflection film, the multilayer comprising aluminum oxide layers having a high refractive index and silicon oxide layers having a low refractive index and being produced by forming the aluminum oxide film first on and in contact with the surface of a quartz substrate, then the silicon oxide film thereon, and further alternately forming the aluminum oxide films and the silicon oxide films.
An optical element for use in shorter-wavelength laser devices such as an ArF excimer laser has been attempted to be produced from fluorite (calcium fluoride) as a substrate material.
However, when an optical element is produced by forming a multilayer film on a fluorite substrate by means of for example sputtering, color centers are generated. As a result, desired spectral characteristics cannot be obtained. Japanese Patent Application Laid-Open Nos. 4-228560 and 5-188203 disclose optical elements in which a 52.0 nm thick silicon oxide film is first formed on a fluorite substrate and then aluminum oxide and silicon oxide films are successively formed thereon, or in which a 62 nm thick silicon oxide film is first formed on a fluorite substrate and then magnesium oxide and silicon oxide films are successively formed thereon.
However, color centers are also generated even in the optical elements produced by these techniques, and thus desired spectral characteristics cannot be obtained.